1. Field of the Invention
This invention relates to a process for obtaining purified silicon. More particularly, it relates to a process for preparing highly purified silicon in high volume, at low cost, suitable for use in fabricating silicon solar cells and other semiconductor devices. Most especially, it relates to a process which will prepare truly non-crystalline, amorphous silicon material having a low level of undesired impurities in sheet form.
2. Description of the Prior Art
Due to increased costs and increasingly unacceptable effects on the environment from other energy sources, there has recently developed a greater interest in the use of photovoltaic silicon solar energy cells as a means for the large scale production of electrical energy. Such silicon solar energy cells are presently well known in the art. However, present processes both for obtaining silicon of sufficient purity to fabricate such devices and present processes for fabricating the devices themselves in the silicon are costly enough to preclude the utilization of such solar energy cells except in space satellites and other equipment intended for remote locations where conventional techniques for generating electricity are unavailable.
Most of the semiconductor grade silicon produced in the United States today is obtained by the reaction of metallurgical grade silicon with anhydrous hydrochloric acid at about 325.degree. C in a fluidized bed to produce trichlorosilane. The trichlorosilane is purified by distillation techniques, then converted into semiconductor grade polycrystalline silicon by thermal rearrangement. The polycrystalline silicon is then formed into ingots of single crystal silicon in a Czochralski furnace. The single crystal ingots are then sliced, lapped and polished to give silicon wafers suitable for fabrication of silicon solar cells and other semiconductor devices.
In the case of integrated circuits, discrete transistors, and similar devices, these techniques for preparing semiconductor grade silicon in the form of wafers have proved to be eminently suitable. The subsequent process steps for forming such devices tend to be complex and labor intensive. Also, the quantity of silicon used per such device is quite small. Therefore, the cost of providing the semiconductor silicon grade wafers typically does not amount to a high proportion of the cost of making such devices.
However, the fabrication of silicon solar cells represents a quite different situation. A much larger volume of purified silicon than is presently being produced will be needed for silicon solar cells to produce a significant amount of even the present requirements of electrical energy. It has been estimated that an area of silicon solar cells roughly equal to the area of all of the paved road in the United States today would be necessary to duplicate the existing conventional and nuclear electrical generating capacity in the United States. Further, processes for making silicon solar energy cells tend to be less complex than most integrated circuit processes. Thus, the cost of providing raw silicon substrates represents a higher proportion of solar energy cell cost than it does of integrated circuit cost.
The present cost for manufacturing semiconductor grade silicon is about $60.00 per kilogram. The cost of producing this raw material must be reduced at least to about $10.00 per kilogram in order to have any realistic chance of large scale successful use of silicon solar cells as a photovoltaic energy source. In addition, the overall cost of producing solar cells must be reduced by about two orders of magnitude in the next decade to be cost competitive with other methods for producing electrical energy. This can only be achieved by a continuous flow, highly automated process. Many of the steps presently used in the fabrication of semiconductor grade silicon are incompatible from a continuous process point of view. Thus, modification or straightforward automation of the present production processes for semiconductor grade silicon do not have the potential of reaching the required cost reduction goal. While U.S. Pat. No. 2,840,588 teaches that polymerized silicon difluoride can be pyrolized to produce silicon, it contains no indication that such a technique would be advantageous for large scale production of silicon.
A related problem area is the energy consumption required for the production of silicon solar cell arrays with present techniques. Currently, the energy consumed in the production of metallurgical grade silicon, its purification to semiconductor grade silicon, the fabrication of silicon solar cells in the silicon, and the assembly of the solar cell arrays amounts to about forty years' worth of electricity generation from the resulting solar cell array. This energy consumption must be reduced by a factor of ten to twnety times in order for the energy payback period to approach the goal of about 10 to 20 percent of the expected lifetime of twenty years for silicon solar cells.
For some time, investigators in the semiconductor field have speculated about the desirability of truly amorphous silicon in the fabrication of semiconductor devices. More recently, D. E. Carlson, et al, "Amorhous Silicon Solar Cell", Applied Physics Letters 28, 671-673 (1976), reports that solar cells fabricated in amorphous silicon have an enhanced efficiency over solar cells fabricated in single crystal silicon. The amorphous silicon used by Carlson et al was made from a glow discharge in silane, a process not amenable to high volume production at low cost. Amorphous silicon has the form of a glassy solid with no crystal structure. It thus does not exhibit any diffraction peaks when scanned on an x-ray deffractometer, nor does it reveal any structural features when examined under a scanning electron microscope, even at a magnification of 100,000 X.
Thus, while the art of preparing semiconductor grade silicon is a well developed one, there remains a need for further development of a basic process which will meet the need for high volume production at low cost of purified silicon, suitable for silicon solar cell applications, as well as for a process which will produce amorphous silicon in volume.